Brucellosis History and Biology

Brucellosis is a highly contagious bacterial disease of both animals and humans that has been recognized since the nineteenth century. A cooperative state-federal brucellosis eradication program has existed for more than seventy years because of the disease’s economic impact on cattle ranchers and because it can be a serious human disease. This program has nearly eliminated brucellosis in domestic livestock, but the disease still exists in free-ranging elk and bison in the greater Yellowstone area, including northwest Wyoming. Brucellosis is not found in wildlife anywhere else in the state.

Brucellosis was probably introduced into the Yellowstone area from infected bison that were transplanted into Yellowstone National Park from a brucellosis-infected cattle ranch. In addition, elk likely contracted brucellosis when they shared feed with infected cattle in and near Yellowstone in the early 1900s.

There are several species of the Brucella bacterium; Brucella abortus is the species that infects elk, bison, and cattle. The current taxonomic scheme recognizes eight strains: B. abortus types 1 and 4 are probably the most common isolates from elk and bison in the greater Yellowstone area.

Brucellosis Infection of the female reproductive tract results in abortion. A cow usually aborts her first calf following infection; a few cows will abort their second, or even third, calves as well. Fetuses delivered near term often are stillborn or fail to thrive due to overwhelming Brucella infection. The male reproductive tract (testes, seminal vesicles, prostate) can also be infected. Infection of the bone or joint membranes results in lameness that may make the animal more susceptible to predation.

The most common route of transmission is thought to be oral. Elk, bison, and most other ungulates lick newborn young, whether the youngster is alive or dead, and whether it is one of their own offspring or not. They often eat placentas, fetal sacs, and even stillborn young. This instinctive reaction to a birth gives Brucella a perfect avenue for infecting new animals. Licking or eating an infected fetus or placenta, licking the vulva of an infected female that has just given birth, consuming any of the fluids that leave an infected female at birth— any of these is enough to transmit brucellosis to another animal.

Under cool, moist conditions, Brucella bacteria can survive for more than 100 days in the environment. An elk or bison that consumes feed or water contaminated by vaginal discharges or fetal membranes may develop brucellosis. Treatment of the disease in wild animals is difficult because it requires multiple drugs administered daily for several weeks.

The Wyoming Game and Fish Department tests elk for brucellosis at many of its feedgrounds. It also gathers blood samples from elk that are thought not to winter regularly on feedgrounds— these samples are taken from elk killed by hunters. The Wyoming Livestock Board and the USDA Animal and Plant Health Inspection Service regularly test cattle throughout the state.

Blood testing shows the proportion of animals that have been exposed to brucellosis and developed antibodies— veterinarians call this proportion “seroprevalence.” This testing doesn't necessarily mean that the animal can transmit the disease. Another test is used to culture the bacteria from tissue samples— a positive case or "culture positive," indicates that the animal actually harbors the bacteria and may be able to transmit to other animals.

Managing Brucellosis

Managing brucellosis in elk, wild bison, and cattle in the greater Yellowstone area is one of the most complicated and contentious wildlife management issues in North America. Often called a “political disease,” brucellosis affects both livestock and wildlife, resulting in social, economic, biological, and political complications that are a constant challenge for wildlife and livestock officials.

Many interest groups— state and federal wildlife and agricultural officials, hunters, ranchers, outfitters, conservationists, landowners, and the general public— are touched by this disease. Managing brucellosis requires a cooperative, multi-pronged approach that uses new techniques, research, and progressive thinking that are, at times, controversial.

Over the last thirty years, a coalition of government agencies, private-sector organizations, and individuals has mounted a growing effort to solve the brucellosis problem. Together, these groups have built an aggressive program of vaccination, conducted research on new vaccines, and managed land to provide better habitat for wildlife. Though Wyoming’s brucellosis problem is far from being solved, these efforts have made a significant difference in the scope and impacts of the disease. Continued research, management actions, and cooperation by all parties involved promises even more progress in the future.